1. Field of the Invention
The present invention relates to a III-V compound semiconductor expressed by the general formula InaGabAlcN (where a+b+c=1, 0≦a≦1, 0≦b≦1, 0≦c≦1).
2. Description of the Related Art
III-V compound semiconductors expressed by the general formula InaGabAlcN (where a+b+c=1, 0≦a≦1, 0≦b≦1, 0≦c≦1) can be used as materials for high efficient light emitting devices ranging from the ultraviolet to the visible region of the electromagnetic spectrum, since their direct band gap corresponding to ultraviolet to red is adjustable by varying the composition of the group III elements. Furthermore, since these compound semiconductors have a large band gap compared with commonly used semiconductors such as Si and GaAs, in theory it is possible to fabricate electronic devices having excellent environmental resistance by utilizing the characteristic that they can retain the property as semiconductors at such high temperatures that make conventional semiconductors inoperable.
In the case of such compound semiconductors, however, since it is extremely difficult to grow a large crystal because of their very high vapor pressure near the melting point, it is not possible to obtain a crystal of practical size that can be used as the substrate for semiconductor device fabrication. Accordingly, in the fabrication of the compound semiconductor, usually sapphire, SiC, or other material having a similar crystal structure to that of the compound semiconductor, and capable of producing a large crystal, is used as the substrate on top of which the compound semiconductor is epitaxially grown. Using such a method, it has become possible to obtain a crystal of the compound semiconductor of relatively good quality. Even then, it is difficult to reduce crystal defects resulting from the difference in lattice constant or thermal expansion coefficient between the substrate material and the compound semiconductor, and a defect density of about 108cm−2 or more usually results.
On the other hand, there has been reported a technique for obtaining a compound semiconductor with reduced defect density, using as the base the compound semiconductor having a high crystal defect density such as described above (Jpn. J. Appl. Phys., Vol. 36, page L899, 1997). That is, the high defect density compound semiconductor (hereinafter sometimes called the base crystal) is covered with a SiO2 pattern, leaving therein microscopic openings, and a second crystal growth is performed on top of that (hereinafter, the second crystal growth may be called the regrowth). An outline of this method will be described with reference to FIG. 1.
First, in the early stage of the regrowth, no crystal growth occurs on the pattern, but crystal growth occurs only in the openings, that is, selective growth occurs. When the crystal growth further continues from this stage, the crystal grown in each opening spreads over the pattern, resulting in a structure burying the pattern therebelow. Though steps remain on the regrown crystal surface immediately after the pattern burying, the steps on the regrown surface are gradually smoothed as the crystal growth progresses and, finally, a flat crystal surface can be obtained.
So far, the following two methods are reported as viable methods for reducing crystal defects in the compound semiconductor by the fabrication of the above-described buried structure. The two methods are a hydride vapor phase epitaxy method (hereinafter sometimes called the HVPE method) and a metal organic vapor phase epitaxy method (hereinafter sometimes called the MOVPE method). These methods, however, have involved the following problems.
First, in the case of the HVPE method, it is known that the compound semiconductor grown over the pattern is oriented at a slightly different angle than the base crystal while the crystal grown over the opening has the crystal orientation aligned with that of the base crystal (Appl. Phys. Lett., Vol. 73, page 481, 1998). Accordingly, the crystal grown over the pattern and the crystal grown over the opening are not aligned in crystal orientation, their interface forming a so-called low angle grain boundary where many edge dislocations are contained. As the regrown crystal increases in thickness, the crystal orientation gradually aligns, but the film thickness where edge dislocations do not occur is required to be about 60 μm or greater. Growing such a thick film not only consumes much time, but also involves the problem that the distortion due to the difference in thermal expansion coefficient between the regrown crystal and the substrate crystal increases. The internal distortion of the substrate causes deformation of the substrate, which in turn causes a problem in crystal growth and, hence, a fabrication problem in the usual semiconductor process.
An object of the present invention is to provide a III-V compound semiconductor in which the occurrence of low angle grain boundaries is suppressed.